Updating SpoonOS docs for easier onboarding for beginner using MacOS (using python3 command instead of python)

[nishenmene]

Suggested update for clarity on the SpoonOS docs: https://xspoonai.github.io/docs/getting-started/installation/

On macOS, it is better to use the python3 command instead of python. As the python command doesn't work on the new ARM MacBooks.

So, for beginner coders, it would be easier to add to the documentation that it is best to try python3 -m venv spoon-env and python3 test_installation.py

Thanks a lot. :-)